Initial impressions: it's early days but this thing is just oozing for customisation. The fact it's written in JS means writing packages is pretty simple compared to Python for ST; we're going to see TONNES of addons for it. I can already think of 2-3 I'd like to build.
The one thing that really excites me is how people getting into programming could start off with a pretty basic editor and add and write packages as they hone their skills, ending with something incredible powerful and tailored to them. In fact I'd really love to see a super customised starter version for children / beginners that acts as a learning aid as well as a text editor.
No doubt there's going to be haters in the vim / emacs / ST / insert favourite editor here camp but I really hope this works out for the Github guys (and I have a feeling it will).